The treatment of industrial water systems, particularly cooling water systems, has over the past 25 years been subject to significant changes. The most prominently recognized treatment for cooling water systems came in the form of Betz Laboratories, Inc.'s Dianodic.RTM. and Zinc-Dianodic product lines. These lines made use of the exceptional capacity of the chromate component in inducing the formation of a passive oxide film, believed to be primarily gamma-ferric oxide, on the metallic surfaces which provided protection against corrosion. The chromates, when used in conjunction with polyphosphates, zinc and in some cases orthophosphates, provided protection which until recently was basically unduplicatable with other treatments. In this regard, U.S. Pat. Nos. 2,711,391; 2,793,932; 2,848,299; 2,872,281; and 2,900,222 can be noted.
With the advent of Federal, State and Municipal environmental controls, however, chromate became suspect for its environmental impact on streams, ponds, lakes, etc., where it might be discharged. Some industries, particularly the petroleum refining, petrochemical, chemical and steel industries, because of each's awareness of the excellent passivation provided by the chromates through the formation of a passive oxide film, decided to continue to use the chromate treatments, with the attendent high capital expenditures for either chromate removal or recovery systems, or for disposing of reduced chromate obtained by the natural treatment of effluents. Other industries, on the contrary, have utilized treatments which avoided the use of chromates. Since chromates are superb corrosion inhibitors, systems such as cooling water systems, were maintained in the acid range, thereby avoiding the pH's where calcium carbonate and magnesium carbonate normally form and precipitate.
With the prohibitions relative to the use of chromate, treatment programs were utilized which made use of the controlled precipitation of calcium phosphate and/or carbonate to form a protective barrier on the surface of the metallic parts (generally ferrous metals) to provide corrosion protection through cathodic action. These programs utilized orthophosphates, polyphosphates, phosphonic acid compounds and their salts. Although these programs were reasonably successful, they did not provide the protection established by the passive oxide film induced by chromate treatments. Moreover, these programs, because of the need for narrow ranges of pH control, were virtually unforgiving, i.e., if concentration of the calcium ion, the phosphate and phosphonate were excessive for a particular pH, uncontrolled precipitation occurred, which on many occasions blocked the metal pipes which the programs were attempting to protect. Control of the operating parameters and conditions was extremely critical.
These treatments, although the best available at the time, did provide reduced heat transfer because of the deposited materials, but also negatively effected production because they did impede flow, for example, of the cooling water. Each of these factors had a direct effect on energy costs, since more energy was required to provide commensurate production to that achieved when chromate was used.
Typical of the chromate-free treatments are those specified in Vogt et al U.S. Pat. No. 3,837,803, which is hereby incorporated herein in its totality by reference thereto. One of the objects of this invention is to improve upon the invention of the above-identified patent.
Until the advent of the present invention the use of the passivated oxide film for corrosion protection has, with the exception of those systems using chromate, been limited since no practical and effective manner has been discovered to provide such in the absence of chromate. Inorganic phosphates, although having the capacity, could not be used effectively. The present inventors, after much investigation, have discovered a treatment(s) which has successfully established the much desired but elusive passive oxide film (also believed to be a gamma-ferric oxide film) on ferrous-based metallic surfaces in contact with aqueous systems, and in particular cooling water systems.
The present inventors discovered that if the aqueous medium contained in or being conveyed by a metallic system was first adjusted to a pH of 5.5 or above, & secondly measured to assure a particular calcium ion content, that the illustrative passive oxide film is formed upon the metallic surfaces to thereby protect such from corrosion through the use of a treatment comprised of an inorganic orthophosphate such as those listed in U.S. Pat. No. 3,837,803 in column 5, lines 1 through 34, together with a water-soluble polymer composed primarily of moieties derived from acrylic acid (or its water-soluble salts) and moieties derived from an hydroxy lower alkyl acrylate. Additional protection is obtained by including in the treatment water-soluble polyphosphates and/or water-soluble organo-phosphonic acid derivatives (or salts thereof). The organo-phosphonic acid derivatives contemplated for use in accordance with the present invention are those disclosed in columns 5 through 9 of U.S. Pat. No. 3,837,803. The polyphosphates operable are those generally used for corrosion inhibition purposes and which will be described more fully later herein.
While the treatments of the invention are effective alone, it is desirable to where copper corrosion is a problem to include copper corrosion inhibitors such as benzotriazole, mercaptobenzothiazole, mercaptobenzothiol, totyltriazole, etc., in the composition or in the treatment.